Various optical scanners are known for such applications as data storage, bar code reading, image scanning (surface definition, surface characterization, robotic vision), and lidar (fight detection and ranging). Referring to FIG. 1, a prior art scanner 50 generates a moving spot of light 60 on a planar target surface 10 by focusing a collimated beam of light 20 through a focusing lens 40. If the assembly is for reading information, reflected light from the constant intensity spot 60 is gathered by focusing lens 40 and returned toward a detector 32. To write information, the light-source is modulated. To cause the light spot 60 to move relative to the surface 10, either the surface 10 is moved or the scanner 50 is moved. Alternatively, the optical path could have an acousto-optical beam deflector, a rotating prism-shaped mirror, or a lens driven galvanometrically or by piezoelectric positioners. Scanners also fall into two functional groups, raster and vector. Both types generally use the same types of beam deflection techniques.
Higher-speed raster scanners use either spinning prism-shaped (polygonal cross-sectioned) mirrors or multifaceted spinning holograms (hologons). Performance parameters for these conventional beam deflection techniques are listed in Table 1. The discrete optics in these devices are generally attended by high costs for mass manufacture, assembly, and alignment.
(from The Photonics Design and Applications Handbook 1993, Laurin Publishing Co., Inc., p. H449)
The performance parameters listed in Table 1 assume different levels of importance depending on the optical scanning application. For raster scanning to cover extended surface areas, the emphasis is on speed, area resolution, and scan efficiency. Wide bandwidth is needed if the surface is to be color-scanned. For applications requiring vector scanning of precise paths at high resolution, the optical system typically uses a monochromatic, focused spot of light that is scanned at high speed with low wavefront distortion and low cross-axis error. Optical data storage has been a prime application of this type of optical scanning.
In optical data storage media, information is stored as an array of approximately wavelength-size dots (cells) in which some optical property has been set at one of two or more values to represent digital information. Commercial read/write heads scan the media with a diffraction-limited spot, typically produced by focusing a collimated laser beam with a fast objective lens system as shown in FIG. 1. A fast objective lens, one with a high numerical aperture, achieves a small spot size by reducing Fraunhofer-type diffraction. The spot is scanned by moving an assembly of optical components (turning mirror, objective lens, position actuators) over the optical medium, either along a radius of a disc spinning under the spot or across the width of a tape moving past the head. The assembly moves in one dimension along the direction of the collimated laser beam. As the disk spins or the tape feeds, the line of bit-cells must be followed by the spot with sufficient precision to avoid missing any bit cells. The fine tracking is achieved by servo mechanisms moving the objective lens relative to the head assembly. An auto-focus servo system is also necessary to maintain the diffraction limited spot size because the medium motion inevitably causes some change in the lens/medium separation with time. Proper focus adjustment is possible because the medium is flat and smooth. Such a surface reflects incident light in well-defined directions like a mirror. Light reflected from the medium is collected by focusing optics and sent back along the collimated beam path for detection.
Scanning by several spots simultaneously is used to achieve high data rates through parallelism in one known system called the CREO(copyright) optical tape system.
The reading of optically stored data is a prime application example of this type of optical scanning. Commercial read/write heads for optical data storage systems scan with a diffraction-limited light spot, typically produced by focusing a collimated laser beam with a fast objective lens system as shown in FIG. 1. The spot is scanned by moving an assembly of optical components (turning mirror, objective lens, position actuators) over the optical storage medium, either along a radius of a disc spinning under the spot or across the width of a tape moving through the head. The assembly moves in one dimension along the direction of the collimated laser beam. Light reflected from the storage medium is collected by the focusing optics and sent back along the collimated beam path. It is diverted out of the source path by a beam splitter 31 for routing to a detector 32. However, because of the collimated beam optical design of this system, light entering the return path from areas outside the scanning spot can propagate some distance back toward the detector before the angular displacement is transformed into sufficient spatial displacement to be caught by an aperture stop. This extraneous light is more of a problem in a multiple spot system in which several areas of the scanned surface are illuminated at once, and crosstalk between adjacent and nearby spots is likely. The use of discrete optical components in such devices to eliminate this effect, poses great difficulty and cost for mass-manufacture because of the requirement of precise optical alignment of components.
One scanning device that avoids reliance on discrete optical elements to achieve scanning is described in U.S. Pat. No. 4,234, 788. In this scanner, an optical fiber is supported rigidly at one end in a cantilevered fashion. The supported end of the fiber is optically coupled to a light emitting diode or photo diode for transmitting or receiving light signals, respectively. The fiber is free to bend when a force is exerted on it. The fiber can thus be made to scan when light from the light-emitting diode emanates from the tip of the fiber as the fiber is forced back and forth repeatedly. To make the fiber wiggle back and forth, an alternating electric field, generally perpendicular to the axis of the fiber, is generated. The fiber is coated with a metallic film. A charge is stored on the film, especially near the tip, by forming a capacitance with a metallized plate oriented perpendicularly to the fiber axis (optically at least partly transparent). The stored charge makes the fiber responsive to the electric field.
A drawback of this device is the limit on the speeds with which the fiber can be made to oscillate. The device requires a series of elements to move the fiber: an external field-generating structure, a DC voltage source to place charge on the fiber coating, and an AC source to generate the external field. Another drawback of this prior art mechanism is the inherent problem of stress fractures in the fiber optics. Bending the fiber repeatedly places serious demands on the materials. Problems can wise due to changes in optical properties, changes in the mechanical properties causing unpredictable variation in the alignment of the plane followed by the bending fiber, the amplitude of vibration, the natural frequency of vibrations, and structural failure. Still another limitation is imposed by the need to place a conductor between the fiber tip and the optical medium to form the capacitance. This places another optical element between the fiber tip and the scanned surface and makes it impossible to sweep the tip very dose to the scanned surface as may be desired for certain optical configurations.
Another prior art scanning device is described in U.S. Pat. No. 5,422, 469. This patent specification describes a number of different devices to oscillate the end of an optical light guide or optical fiber. One embodiment employs a piezo-electric bimorph connected to the free end of a device to which the free end of an optical fiber and a focusing lens are attached. Reflected light is directed back through the fiber to a beam splitter which directs the reflected light out of the bidirectional (outgoing/return) path at some point along the fiber remote from the source of light. The above embodiment uses a simpler prime mover, a piezo-electric bimorph However, the need for a focusing lens attached to the end of the fiber, by increasing the mass, imposes difficult practical requirements for high speed oscillation of the fiber. In addition, to achieve very small projected spot size requires a high numerical aperture at the output end of the focusing optics. It is difficult to achieve this with the conventional optics contemplated by the ""469 disclosure. Furthermore, the reciprocation of the fiber as described in the ""469 patent requires a multiple-element device. Friction between the motor and the fiber can cause changes in the optical properties of the fiber, and mechanical changes in the motor, the fiber, or the interface, that result in changes (which may be unpredictable) in the amplitude of oscillation or the resonant frequency of the motor-fiber combination (which might generate, or be susceptible to, undesired harmonics). Also, the process of assembly of such a combination of a motor and a fiber presents problems. Ideally, for high frequency operation, the device would be very small.
Common to all storage/retrieval devices is the need for greater and greater data rates. Increases in speed have been achieved by increasing the speed of scanning. However, there are practical limits, particularly with regard to the writing operation, relating to physical properties inherent in the optical media.
Also common to the applications of optical scanning technology is the need for great precision in the focus of the scanning light source and the return signal.
A multiple channel scanning device has a scanning head with multiple columns of apertures that emit light which is imaged by a lens onto the surface of a recorded medium. Light returned from the medium is imaged back onto the apertures and conducted to detectors. In a preferred embodiment, the scanning head is rapidly oscillated (may be on the order of 100 kHz rate), in a direction parallel to the columns. The medium is moved in a direction perpendicular to the columns so that the same recorded regions pass beneath successive columns of apertures. The data from the detectors is image-processed to improve the quality of data-reading using the successive readings from the same data regions. This allows errors to be corrected and throughput to be improved. In an alternative embodiment, scan spots are swept over nearly the same, or the same, regions to achieve oversampling.
According to an embodiment, the invention provides a scanning device for scanning a target surface with data written on it. The data is arranged in adjacent data cells on the target surface. Each of the cells has one of a set of possible configurations representing data. For example, a cell could be highly reflective to represent a xe2x80x9c1xe2x80x9d and less reflective to represent a xe2x80x9c0.xe2x80x9d The scanning device has a read/write head, with at least one laser source, that transmits light to an array of output apertures from which light is emitted. The light returned from the surface is received through an array of input apertures. The read/write head and the target surface are mutually supported to move relative to each other to scan the target surface. The array of output apertures is arranged such that some scan substantially the same cells of the target surface. The read/write head includes detectors that detect the returned light and send resulting signals to an image processor. The image processor generates an estimate of a configuration of each cell from the redundant or quasi-redundant data and generates a signal stream representing the estimate. In a variation, the output apertures are coaxial with the input apertures. In another variation, the read/write head has an optoelectronic chip with internal light guides formed in it, each of the light guides being connected to one of the output apertures. In still another variation, the optoelectronic chip has at least one optical switch to modulate an output of either a reading laser source or a writing laser source to allow the scanning device to write data as well as read it. The light sources of the invention, for writing purposes, are, preferably, modulated by optical switches that selectively direct the output between a write output aperture and another direction leading ultimately to dissipation of energy of the writing laser source. This way, the writing laser source can operate continuously during writing. Multiple reading laser sources may be connected to an array of light guides interconnected to split light from the multiple reading laser sources into multiple paths, each connected to a one of the output apertures. The array of light guides may be interconnected with respective optical switches controlled by a controller programmed to cause the laser output to be shared among multiple output apertures by alternately shunting the laser output to a first fraction of the output apertures and shunting the laser output to second fraction of the output apertures. The fractions could constitute just a single aperture.
According to another embodiment, the invention provides a scanning device for scanning a target surface with data written on it. The data is arranged in columns of adjacent data cells on the target surface. Each of the columns of data cells has one of a set of possible configurations representing data as discussed above. The device has a read/write head with an array of input apertures arranged in successive columns such that each of the columns receives light from the same one of the columns of data cells. There is at least one detector connected to detect light received by the array of input apertures. The detector generates a signal indicating an estimate of one of the possible configurations by combining information derived from light received by all of the successive columns. In a variation, the detector combines the information by detecting light from each of the columns and synthesizing an improved estimate of the one of the possible configurations from the combination of signals generated.
According to still another embodiment, the invention provides a scanning device for scanning a target surface that has data written thereon, the data is arranged in columns of adjacent data cells on the target surface. Each of the columns of data cells has one of a set of possible configurations representing data. The device has a scanning head with an array of input apertures arranged in successive columns so each of the columns receives light returned from the columns of data cells passing under it. Also at least one detector is connected to detect light received by the array of input apertures. The detector generates a signal indicating an estimate of one of the possible configurations by combining information derived from light received by all of the successive columns. In a variation, the scanning head has at least one laser connected to conduct light so that it is emitted from the array of input apertures. In this way, the array of input apertures functions as an array of output apertures from which light is emitted. In another variation, an imaging optical element positioned between the scanning head and the target surface images light emitted from the output apertures onto the target surface. The light from the same one of the columns of data cells is light emitted from the array of output apertures, returned from the target surface, and imaged by the imaging optical element back onto the input apertures. In another variation, there is an array of output apertures, each being respective of one of the array of input apertures. Also, the scanning head includes a light guide leading from each of the input apertures to the respective detector.
According to still another embodiment, the invention provides a method of reading data from a recorded surface that has successive columns of data cells. The successive columns have at least one row of the data cells. The method has the following steps: Moving the recorded surface such that light from a first output aperture is focused onto a first of the successive columns. Receiving light returned from the recorded surface responsively to the first step of moving. Detecting light returned from the recorded surface and storing a first result thereof Moving the recorded surface such that light from a second output aperture is focused onto the first of the successive columns. Receiving light returned from the recorded surface responsively to the second step of moving. Detecting light returned to the first input aperture and storing a second result thereof Calculating data represented by the first of the respective columns responsively to a computed combination of the first and second results. In a variation of the method, in the first step of receiving, light is received at a first input aperture corresponding to the first output aperture. In addition, in the second step of receiving, light is received at a second input aperture corresponding to the second output aperture.
According to still another embodiment, the invention provides a method of reading data from a recorded surface with successive columns of data cells. The successive columns comprise at least one row of the data cells. The method has the following steps: Moving the recorded surface such that light from a first output aperture is focused onto a first of the successive columns. Receiving light returned from the recorded surface responsively to the first step of moving. Detecting light returned from the recorded surface and storing a first result thereof Moving the recorded surface such that light from a second output aperture is focused onto the first of the successive columns. Receiving light returned from the recorded surface responsively to the second step of moving. Detecting light returned to the first input aperture and storing a second result thereof Calculating data represented by the first of the respective columns responsively to a computed combination of the first and second results. In a variation of the method, in the first step of receiving, light is received at a first input aperture corresponding to the first output aperture. In addition, in the second step of receiving, light is received at a second input aperture corresponding to the second output aperture.
According to another embodiment, the invention provides a scanning device for scanning a medium with data written on it. The data is arranged in columns of adjacent data cells on the target surface. Each of the columns of data cells has one of a set of possible configurations representing data The scanning device has a scanning head with an array of input apertures arranged in successive columns such that each of the columns receives light from the same one of the columns of data cells. In addition, at least one detector is connected to detect light received by the array of input apertures. The detector generates a signal indicating an estimate of a one of the possible configurations by combining information derived from light received by all of the successive columns. There is a frame connected to the scanning head. The medium is attachable to the frame such that the medium is movable relative to the read/write head. As a result, the media moves in a first direction relative to the read/write head. An oscillating motor connected between the frame and the read/write head oscillates the scanning head relative to the medium. As a result a spacing of the input apertures may exceed a spacing of the adjacent cells while still permitting light returned from substantially all of the adjacent cells to be detected by the detector. In a variation, the medium is moved continuously in the first direction at a constant speed. In another variation, the direction of an oscillation of the read/write head has a component substantially perpendicular to the first direction. In still another variation, the scanning head includes at least one laser connected to conduct light so that it is emitted from array of input apertures. As a result, the array of input apertures functions as an array of output apertures from which light is emitted. In still another variation there is an imaging optical element (e.g., a lens system) positioned between the scanning head and the target surface to image light emitted from the output apertures onto the target. The light from the same one of the columns of data cells is emitted and returned from the array of output apertures. This light is imaged by the same imaging optical element back onto the input apertures. In another variation, there is an array of detectors, each being respective of one of the input apertures. The scanning head includes a light guide leading from each of the input apertures to the respective detector.
The invention provides an essential component in an optoelectronic chip designed to direct the flow of light and modulate the light output in a multi-channel optical scanning head. The invention leads to a reliable, robust, manufacturable, low-cost component for optical scanning devices used for optical data storage, bar code readers, image scanning for digitization or xerography, laser beam printers, inspection systems, densitometers, and 3-dimensional scanning (surface definition, surface characterization, robotic vision). Speed and accuracy are enhanced through the use of image processing techniques applied to redundant and partly redundant data.